The effects of ionizing and ultraviolet radiation on nucleic acids and proteins and their constituents are being studied. The modification of radiation damage in DNA by cancer chemotherapy agents of the intercalating and alkylating types is of interest, since such information may be useful in radiation therapy. The present report includes electron spin resonance (e.s.r.) studies of spin-trapped free radicals produced by gamma-radiolysis of amino acids, peptides, nucleic acid bases, and dihydropyrimidines. The direct UV photolyses of dihydropyrimidines were investigated. The benzoylperoxide induced photolyses of carboxylic acids, amino acids, peptides, and pyrimidines were studied in dimethylsulfoxide as solvent.